Understanding Program Statements in Java

1. INF 523Q Chapter 3: Program Statements

2. Program Statements • Chapter 3 focuses on: • the flow of control through a method • decision-making statements • operators for making complex decisions • repetition statements • software development stages

3. Flow of Control • Unless indicated otherwise, the order of statement execution through a method is linear: one after the other in the order they are written • Some programming statements modify that order, allowing us to: • decide whether or not to execute a particular statement, or • perform a statement over and over repetitively • The order of statement execution is called the flow of control

4. Conditional Statements • A conditional statement lets us choose which statement will be executed next • Therefore they are sometimes called selection statements • Conditional statements give us the power to make basic decisions • Java's conditional statements are the if statement, the if-else statement, and the switch statement

5. The condition must be a boolean expression. It must evaluate to either true or false. if is a Java reserved word If the condition is true, the statement is executed. If it is false, the statement is skipped. The if Statement • The if statement has the following syntax: if ( condition ) statement;

6. The if Statement • An example of an if statement: if (sum > MAX) delta = sum - MAX; System.out.println ("The sum is " + sum); First, the condition is evaluated. The value of sum is either greater than the value of MAX, or it is not. If the condition is true, the assignment statement is executed. If it is not, the assignment statement is skipped. Either way, the call to println is executed next. • See Age.java (page 112)

7. Age.java • //****************************************************************** • // Age.java Author: Lewis and Loftus • // Demonstrates the use of an if statement. • //****************************************************************** • import cs1.Keyboard; • publicclass Age • { • // Reads the user's age and prints comments accordingly. • publicstaticvoid main (String[] args) • { • finalint MINOR = 21; • System.out.print ("Enter your age: "); • int age = Keyboard.readInt(); • System.out.println ("You entered: " + age); • if (age < MINOR) • System.out.println ("Youth is a wonderful thing. Enjoy."); • System.out.println ("Age is a state of mind."); • } • }

8. condition evaluated true false statement Logic of an if statement

9. Boolean Expressions • A condition often uses one of Java's equality operators or relational operators, which all return boolean results: == equal to != not equal to < less than > greater than <= less than or equal to >= greater than or equal to • Note the difference between the equality operator (==) and the assignment operator (=)

10. The if-else Statement • An else clause can be added to an if statement to make it an if-else statement: if ( condition ) statement1; else statement2; • If the condition is true, statement1 is executed; if the condition is false, statement2 is executed • One or the other will be executed, but not both • See Wages.java (page 116)

11. Wages.java • //****************************************************************** • // Wages.java Author: Lewis and Loftus • // Demonstrates the use of an if-else statement. • //****************************************************************** • import java.text.NumberFormat; • import cs1.Keyboard; • publicclass Wages • { • // Reads the number of hours worked and calculates wages. • publicstaticvoid main (String[] args) • { • finaldouble RATE = 8.25; // regular pay rate • finalint STANDARD = 40; // standard hours in a work week • double pay = 0.0; • System.out.print ("Enter the number of hours worked: "); • int hours = Keyboard.readInt(); • System.out.println ();

12. Wages.java (cont.) • // Pay overtime at "time and a half" • if (hours > STANDARD) • pay = STANDARD * RATE + (hours-STANDARD) * (RATE * 1.5); • else • pay = hours * RATE; • NumberFormat fmt = NumberFormat.getCurrencyInstance(); • System.out.println ("Gross earnings: " + fmt.format(pay)); • } • }

13. condition evaluated true false statement1 statement2 Logic of an if-else statement

14. Block Statements • Several statements can be grouped together into a block statement • A block is delimited by braces ( { … } ) • A block statement can be used wherever a statement is called for in the Java syntax • For example, in an if-else statement, the if portion, or the else portion, or both, could be block statements • See Guessing.java (page 117)

15. Guessing.java • //****************************************************************** • // Guessing.java Author: Lewis and Loftus • // Demonstrates the use of a block statement in an if-else. • //****************************************************************** • import cs1.Keyboard; • publicclass Guessing • { • // Plays a simple guessing game with the user. • publicstaticvoid main (String[] args) • { • finalint MAX = 10; • int answer, guess; • answer = (int) (Math.random() * MAX) + 1; • System.out.print ("I'm thinking of a number between 1 and " • + MAX + ". Guess what it is: "); • guess = Keyboard.readInt();

16. Guessing.java (cont.) • if (guess == answer) • System.out.println ("You got it! Good guessing!"); • else • { • System.out.println ("That is not correct, sorry."); • System.out.println ("The number was " + answer); • } • } • }

17. Nested if Statements • The statement executed as a result of an if statement or else clause could be another if statement • These are called nested if statements • See MinOfThree.java (page 118) • An else clause is matched to the last unmatched if (no matter what the indentation implies)

18. MinOfThree.java • //****************************************************************** • // MinOfThree.java Author: Lewis and Loftus • // Demonstrates the use of nested if statements. • //****************************************************************** • import cs1.Keyboard; • publicclass MinOfThree • { • // Reads three integers from the user and determines the smallest value. • publicstaticvoid main (String[] args) • { • int num1, num2, num3, min = 0; • System.out.println ("Enter three integers: "); • num1 = Keyboard.readInt(); • num2 = Keyboard.readInt(); • num3 = Keyboard.readInt();

19. MinOfThree.java (cont.) • if (num1 < num2) • if (num1 < num3) • min = num1; • else • min = num3; • else • if (num2 < num3) • min = num2; • else • min = num3; • System.out.println ("Minimum value: " + min); • } • }

20. Comparing Characters • We can use the relational operators on character data • The results are based on the Unicode character set • The following condition is true because the character '+' comes before the character 'J' in Unicode: if ('+' < 'J') System.out.println ("+ is less than J"); • The uppercase alphabet (A-Z) and the lowercase alphabet (a-z) both appear in alphabetical order in Unicode

21. Comparing Strings • Remember that a character string in Java is an object • We cannot use the relational operators to compare strings • The equals method can be called on a string to determine if two strings contain exactly the same characters in the same order • The String class also contains a method called compareTo to determine if one string comes before another alphabetically (as determined by the Unicode character set)

22. Comparing Floating Point Values • We also have to be careful when comparing two floating point values (float or double) for equality • You should rarely use the equality operator (==) when comparing two floats • In many situations, you might consider two floating point numbers to be "close enough" even if they aren't exactly equal • Therefore, to determine the equality of two floats, you may want to use the following technique: if (Math.abs (f1 - f2) < 0.00001) System.out.println ("Essentially equal.");

23. The switch Statement • The switch statement provides another means to decide which statement to execute next • The switch statement evaluates an expression, then attempts to match the result to one of several possible cases • Each case contains a value and a list of statements • The flow of control transfers to statement list associated with the first value that matches

24. switch and case are reserved words If expression matches value2, control jumps to here The switch Statement • The general syntax of a switch statement is: switch ( expression ) { case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case ... }

25. The switch Statement • Often a break statement is used as the last statement in each case's statement list • A break statement causes control to transfer to the end of the switch statement • If a break statement is not used, the flow of control will continue into the next case • Sometimes this can be helpful, but usually we only want to execute the statements associated with one case

26. The switch Statement • A switch statement can have an optional default case • The default case has no associated value and simply uses the reserved word default • If the default case is present, control will transfer to it if no other case value matches • Though the default case can be positioned anywhere in the switch, it is usually placed at the end • If there is no default case, and no other value matches, control falls through to the statement after the switch

27. The switch Statement • The expression of a switch statement must result in an integral data type, like an integer or character; it cannot be a floating point value • Note that the implicit boolean condition in a switch statement is equality - it tries to match the expression with a value • You cannot perform relational checks with a switch statement • See GradeReport.java (page 121)

28. GradeReport.java • //****************************************************************** • // GradeReport.java Author: Lewis and Loftus • // Demonstrates the use of a switch statement. • //****************************************************************** • import cs1.Keyboard; • publicclass GradeReport • { • // Reads a grade from the user and prints comments accordingly. • publicstaticvoid main (String[] args) • { • int grade, category; • System.out.print ("Enter a numeric grade (0 to 100): "); • grade = Keyboard.readInt(); • category = grade / 10; • System.out.print ("That grade is "); • switch (category) • { • case 10: • System.out.println ("a perfect score. Well done."); • break;

29. GradeReport.java (cont.) • case 9: • System.out.println ("well above average. Excellent."); • break; • case 8: • System.out.println ("above average. Nice job."); • break; • case 7: • System.out.println ("average."); • break; • case 6: • System.out.println ("below average. You should see the"); • System.out.println ("instructor to clarify the material " • + "presented in class."); • break; • default: • System.out.println ("not passing."); • } • } • }

30. Logical Operators • Boolean expressions can also use the following logical operators: ! Logical NOT && Logical AND || Logical OR • They all take boolean operands and produce boolean results • Logical NOT is a unary operator (it has one operand), but logical AND and logical OR are binary operators (they each have two operands)

31. a true false !a false true Logical NOT • The logical NOT operation is also called logical negation or logical complement • If some boolean condition a is true, then !a is false; if a is false, then !a is true • Logical expressions can be shown using truth tables

32. Logical AND and Logical OR • The logical and expression a && b is true if both a and b are true, and false otherwise • The logical or expression a || b is true if a or b or both are true, and false otherwise

33. a true true false false b true false true false a && b true false false false a || b true true true false Truth Tables • A truth table shows the possible true/false combinations of the terms • Since && and || each have two operands, there are four possible combinations of true and false

34. Logical Operators • Conditions in selection statements and loops can use logical operators to form complex expressions if (total < MAX && !found) System.out.println ("Processing…"); • Logical operators have precedence relationships between themselves and other operators

35. total < MAX && !found false false true false total < MAX false false true true found false true false true !found true false true false Truth Tables • Specific expressions can be evaluated using truth tables

36. More Operators • To round out our knowledge of Java operators, let's examine a few more • In particular, we will examine the: • increment and decrement operators • assignment operators • conditional operator

37. Increment and Decrement Operators • The increment and decrement operators are arithmetic and operate on one operand • The increment operator (++) adds one to its operand • The decrement operator (--) subtracts one from its operand • The statement count++; is essentially equivalent to count = count + 1;

38. Increment and Decrement Operators • The increment and decrement operators can be applied in prefix form (before the variable) or postfix form (after the variable) • When used alone in a statement, the prefix and postfix forms are basically equivalent. That is, count++; is equivalent to ++count;

39. Expression count++ ++count count-- --count Operation add 1 add 1 subtract 1 subtract 1 Value of Expression old value new value old value new value Increment and Decrement Operators • When used in a larger expression, the prefix and postfix forms have a different effect • In both cases the variable is incremented (decremented) • But the value used in the larger expression depends on the form:

40. Increment and Decrement Operators • If count currently contains 45, then total = count++; assigns 45 to total and 46 to count • If count currently contains 45, then total = ++count; assigns the value 46 to both total and count

41. Assignment Operators • Often we perform an operation on a variable, then store the result back into that variable • Java provides assignment operators to simplify that process • For example, the statement num += count; is equivalent to num = num + count;

42. Operator += -= *= /= %= Example x += y x -= y x *= y x /= y x %= y Equivalent To x = x + y x = x - y x = x * y x = x / y x = x % y Assignment Operators • There are many assignment operators, including the following:

43. Assignment Operators • The right hand side of an assignment operator can be a complete expression • The entire right-hand expression is evaluated first, then the result is combined with the original variable • Therefore result /= (total-MIN) % num; is equivalent to result = result / ((total-MIN) % num);

44. The Conditional Operator • Java has a conditional operator that evaluates a boolean condition that determines which of two other expressions is evaluated • The result of the chosen expression is the result of the entire conditional operator • Its syntax is: condition ? expression1 : expression2 • If the condition is true, expression1 is evaluated; if it is false, expression2 is evaluated

45. The Conditional Operator • The conditional operator is similar to an if-else statement, except that it is an expression that returns a value • For example: larger = (num1 > num2) ? num1 : num2; • If num1 is greater that num2, then num1 is assigned to larger; otherwise, num2 is assigned to larger • The conditional operator is ternary, meaning that it requires three operands

46. The Conditional Operator • Another example: System.out.println ("Your change is " + count + (count == 1) ? "Dime" : "Dimes"); • If count equals 1, then "Dime" is printed • If count is anything other than 1, then "Dimes" is printed

47. Repetition Statements • Repetition statements allow us to execute a statement multiple times repetitively • They are often simply referred to as loops • Like conditional statements, they are controlled by boolean expressions • Java has three kinds of repetition statements: the while loop, the do loop, and the for loop • The programmer must choose the right kind of loop for the situation

48. while is a reserved word If the condition is true, the statement is executed. Then the condition is evaluated again. The while Statement • The while statement has the following syntax: while ( condition ) statement; The statement is executed repetitively until the condition becomes false.

49. condition evaluated true false statement Logic of a while loop

50. The while Statement • Note that if the condition of a whilestatement is false initially, the statement is never executed • Therefore, the body of a while loop will execute zero or more times • See Counter.java (page 133) • See Average.java (page 134) • See WinPercentage.java (page 136)